Here are the key quotes from Supreme Court's ruling on prorogation

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- "It is important to emphasise that the issue in these appeals is not when and on what terms the United Kingdom is to leave the European Union. The issue is whether the advice given by the Prime Minister to Her Majesty the Queen ... was lawful."

- "It is not suggested in these appeals that Her Majesty was other than obliged by constitutional convention to accept that advice. In the circumstances, we express no view on that matter. That situation does, however, place on the Prime Minister a constitutional responsibility, as the only person with power to do so, to have regard to all relevant interests, including the interests of Parliament."

- "Although the courts cannot decide political questions, the fact that a legal dispute concerns the conduct of politicians, or arises from a matter of political controversy, has never been sufficient reason for the courts to refuse to consider it."

- "The courts have exercised a supervisory jurisdiction over the decisions of the executive for centuries. Many if not most of the constitutional cases in our legal history have been concerned with politics in that sense."

- The ruling referred to the case of Proclamations, from 1611, in which the court at the time concluded that "the King hath no prerogative, but that which the law of the land allows him".

- "The Prime Minister's accountability to Parliament does not in itself justify the conclusion that the courts have no legitimate role to play. ... The effect of prorogation is to prevent the operation of ministerial accountability to Parliament during the period when Parliament stands prorogued. ... In such circumstances, the most that Parliament could do would amount to closing the stable door after the horse had bolted."

- "The courts have a duty to give effect to the law, irrespective of the minister's political accountability to Parliament. The fact that the minister is politically accountable to Parliament does not mean that he is therefore immune from legal accountability to the courts."

- "If the issue before the court is justiciable, deciding it will not offend against the separation of powers. As we have just indicated, the court will be performing its proper function under our constitution."

- "Although the United Kingdom does not have a single document entitled 'The Constitution', it nevertheless possesses a constitution, established over the course of our history by common law, statutes, conventions and practice. ... Nevertheless, it includes numerous principles of law, which are enforceable by the courts ... In giving them effect, the courts have the responsibility of upholding the values and principles of our constitution and making them effective. ... The courts cannot shirk that responsibility merely on the ground that the question raised is political in tone or context."

- "Time and again, in a series of cases since the 17th century, the courts have protected parliamentary sovereignty from threats posed to it by the use of prerogative powers, and in doing so have demonstrated that prerogative powers are limited by the principle of parliamentary sovereignty."

- "The sovereignty of Parliament would ... be undermined as the foundational principle of our constitution if the executive could, through the use of the prerogative, prevent Parliament from exercising its legislative authority for as long as it pleased ... An unlimited power of prorogation would therefore be incompatible with the legal principle of Parliamentary sovereignty."

- "Ministers are accountable to Parliament through such mechanisms as their duty to answer Parliamentary questions and to appear before Parliamentary committees, and through Parliamentary scrutiny of the delegated legislation which ministers make. By these means, the policies of the executive are subjected to consideration by the representatives of the electorate, the executive is required to report, explain and defend its actions, and citizens are protected from the arbitrary exercise of executive power."

- "That principle (of parliamentary accountability) is not placed in jeopardy if Parliament stands prorogued for the short period which is customary, and as we have explained, Parliament does not in any event expect to be in permanent session. But the longer that Parliament stands prorogued, the greater the risk that responsible government may be replaced by unaccountable government: the antithesis of the democratic model."

- "A prerogative power is ... limited by statute and the common law, including, in the present context, the constitutional principles with which it would otherwise conflict."

- "For the purposes of the present case, the relevant limit upon the power to prorogue can be expressed in this way: that a decision to prorogue Parliament (or to advise the monarch to prorogue Parliament) will be unlawful if the prorogation has the effect of frustrating or preventing, without reasonable justification, the ability of Parliament to carry out its constitutional functions as a legislature and as the body responsible for the supervision of the executive. In such a situation, the court will intervene if the effect is sufficiently serious to justify such an exceptional course."

- "The first question, therefore, is whether the Prime Minister's action had the effect of frustrating or preventing the constitutional role of Parliament in holding the Government to account. The answer is that of course it did.

- "This was not a normal prorogation in the run-up to a Queen's Speech. It prevented Parliament from carrying out its constitutional role for five out of a possible eight weeks between the end of the summer recess and exit day on October 31.

- "Parliament might have decided to go into recess for the party conferences during some of that period but, given the extraordinary situation in which the United Kingdom finds itself, its members might have thought that parliamentary scrutiny of government activity in the run-up to exit day was more important and declined to do so, or at least they might have curtailed the normal conference season recess because of that. Even if they had agreed to go into recess for the usual three-week period, they would still have been able to perform their function of holding the government to account. Prorogation means that they cannot do that."

- "Such an interruption in the process of responsible government might not matter in some circumstances. But the circumstances here were, as already explained, quite exceptional. A fundamental change was due to take place in the Constitution of the United Kingdom on October 31. Whether or not this is a good thing is not for this or any other court to judge. The people have decided that. But that Parliament, and in particular the House of Commons as the democratically-elected representatives of the people, has a right to have a voice in how that change comes about is indisputable."

- "The House of Commons has already demonstrated, by its motions against leaving without an agreement and by the European Union (Withdrawal) (No 2) Act 2019, that it does not support the Prime Minister on the critical issue for his Government at this time and that it is especially important that he be ready to face the House of Commons."

- "We are not concerned with the Prime Minister's motive in doing what he did. We are concerned with whether there was a reason for him to do it."

- "It is impossible for us to conclude, on the evidence which has been put before us, that there was any reason - let alone a good reason - to advise Her Majesty to prorogue Parliament for five weeks. We cannot speculate, in the absence of further evidence, upon what such reasons might have been. It follows that the decision was unlawful."

- "That advice (to prorogue Parliament) was unlawful. It was outside the powers of the Prime Minister to give it. This means that it was null and of no effect."

- "It led to the Order in Council which, being founded on unlawful advice, was likewise unlawful, null and of no effect, and should be quashed. This led to the actual prorogation, which was as if the Commissioners had walked into Parliament with a blank piece of paper. It too was unlawful, null and of no effect."

- "It follows that Parliament has not been prorogued and that this court should make declarations to that effect. We have been told by counsel for the Prime Minister that he will 'take all necessary steps to comply with the terms of any declaration made by the court' and we expect him to do so."

- "However, it appears to us that, as Parliament is not prorogued, it is for Parliament to decide what to do next. There is no need for Parliament to be recalled. Nor has Parliament voted to adjourn or go into recess.

- "Unless there is some parliamentary rule to the contrary of which we are unaware, the Speaker of the House of Commons and the Lord Speaker can take immediate steps to enable each house to meet as soon as possible to decide upon a way forward. That would, of course, be a proceeding in Parliament which could not be called in question in this or any other court."